They dropped it from the original F1 application in preference to the batteries that the other F1 teams were using. A few Porache 911 GT3 cars were modified to use the flywheel. These track only cars didn't have a passenger seat, instead the flywheel was there, spinning round at tens of thousands of revolutions per minute, just next to your 'privates'.
More recently Le Mans cars have had the flywheel although nowadays the business is owned by GKN. Williams sold the business after it was not found to be what they needed for F1. Lots of London busses have the flywheel, it has found its application in stopping busses and getting them back up to speed quickly.
How much power does the flywheel give out? The peak power is about 3/5 of a Nissan Leaf. It is designed to be used in series with the main engine or in parallel. In this way the vehicle can be designed to have a much smaller engine with the flywheel taking care of all acceleration. Multiple flywheels can be used.
The applications the flywheel is sold into are specialist where things get serviced properly. I don't think it will ever be in mainstream automotive because we are giving up on ICE cars and if you have got batteries on the car anyway then you might as well use them for the regen.
Now where I think Williams went wrong was in not making a flywheel that happened to be made from some lithium-ion batteries. So that metric tonne of batteries lugged around on a Tesla, imagine how much potential energy you could store if you flew that around at 36k RPM. You could spin the thing up before leaving the house and not have to use the battery for your first few miles.
I would like to see a purely mechanical version of flywheel, so just gears and a clutch to transfer energy to the flywheel when braking and accelerating away.